1. Field of the Invention
The invention relates to a control apparatus and control method for an internal combustion engine. More specifically, the invention relates to control technology for reducing the amount of HC discharged during startup of the internal combustion engine.
2. Description of the Related Art
During a cold start of an internal combustion engine, fuel atomizes poorly compared to after the engine has warmed up. As a result, unburned HC (Hydro-Carbon) which did not contribute to combustion tends to adhere to the surface of the cylinder wall. When the piston rises in the cylinder, the unburned HC that had adhered to the surface of the cylinder wall is pushed up by the piston and released into the exhaust passage together with combustion gas when the exhaust valve opens. Accordingly, large amounts of unburned HC is contained in combustion gas that is discharged from the internal combustion engine during a cold start, and particularly in combustion gas discharged right before TDC (Top Dead Center), compared with after the engine has warmed up.
A catalyst for removing toxic components in the combustion gas is disposed in the exhaust passage of the internal combustion engine. When the catalyst is cold, however, it is unable to purify the combustion gas sufficiently. As a result, when the catalyst temperature is low and the catalyst is not yet activated, such as during a cold start of the internal combustion engine, unburned HC contained in the combustion gas ends up being released into the atmosphere without being sufficiently purified. Therefore, in order to prevent exhaust gas emissions during a cold start from deteriorating, it is necessary to reduce the actual amount of unburned HC that is discharged from the internal combustion engine.
Japanese Patent Application Publication No. JP-A-2003-120348 discloses one example of technology that reduces the amount of unburned HC discharged during a cold start. The related art disclosed in that publication aims to reduce the amount of unburned HC that is discharged by advancing the closing timing of the exhaust valve with respect to TDC of the intake stroke from engine startup, and closing combustion gas containing large amounts of unburned HC to be discharged in the combustion chamber right before TDC. Japanese Patent Application Publication No. JP-A-2001-159353 and Japanese Patent Application Publication No. JP-A-2001-263050 also disclose related art.
There is a need to reduce the amount of unburned HC that adheres to the surface of the cylinder wall during combustion in the first cycle at startup of the internal combustion engine by burning the fuel that is injected for the initial combustion as completely as possible. Accordingly, it is necessary to sufficiently promote an HC oxidation reaction within the combustion chamber.
During startup, and particularly during a cold start, of the internal combustion engine, however, the temperature and pressure in the combustion chamber are low, opposite of a high temperature-high pressure atmosphere that promotes an HC oxidation reaction. In addition, because the temperature of the surface of the cylinder wall is low, fuel tends to adhere to it. The related art disclosed in Japanese Patent Application Publication No. JP-A-2003-120348 does not take into consideration unburned HC that is produced during combustion in the first cycle at startup. Thus there is still room for improvement with respect to reducing the amount of unburned HC that is discharged.